Dr Simon Harding, of Middlesex University London, who is behind the research, said: "For many young people, dogs are increasingly viewed as a commodity which can be traded up or down like a mobile phone.

"It has become less about whether the dog will fit into family life and more about, 'What will this dog do for me, how much will it make me?'."

He added that "through their reputation for aggression or ability to intimidate", bull breeds "are also used in drug deals, gambling debts and loan-sharking, where their owners do not have recourse to law if the money owed is not paid because his business is illegal".

"The dog says, 'I am here to be taken seriously' - it acts as a 'minder' and a 'heavy' when collecting dues. People believe that possession of an aggressive dog means that the threats posed by such men will be carried out."

The study found the most aggressive dogs could be sold for more than £400, with owners building up their pet's muscles with vitamin supplements and even injecting them with steroids for fights.

Dr Harding interviewed illegal and legal dog owners as well as gang members as part of his research. A 16-year-old boy told him: "It's not just a dog, it's a half bull mastiff and half pit bull. I'll probably get another - we are looking to breed it - and we would get about £2,000 per dog."

The study, which will be presented to the British Sociological Association annual conference, also found there has been a rise of 551% in hospital admissions for dog bites since 1991.

Dr Harding said the growth in numbers raised the risk of attack, particularly for children. He called for animal welfare agencies and the police to work more closely to tackle the issue. Dr Harding added: "Dogs are what we make them, it is humans that are responsible for making dogs either sociable or aggressive."